• Hi Guest!
    You can help ensure that British Car Forum (BCF) continues to provide a great place to engage in the British car hobby! If you find BCF a beneficial community, please consider supporting our efforts with a subscription.
    There are some perks with a member upgrade!
    **Upgrade Now**
    (PS: Subscribers don't see this gawd-aweful banner
Tips
Tips

new used carbs

Share this page

Woodie

Woodie

Jedi Warrior
Offline
Ok I won an auction on ebay, for a set of matched hs2's, the ones with the vacuum port for the EGR. Anyway the DPO said they failed a leak down test, and they did not have the tools to balance them so they switched to weber. They were rebuilt and have about 3 yrs of operation. I did not think leak down test was for carbs. Is he just a DPO?
 
DPO me thinks, unless he was saying that the throtle rods were leaking? However if they only have 3 years on a rebuild your guess is as good as mine.
 
Maybe he meant the pistons were dropping at different rates. This can usually be solved by some fine emery cloth and some patience. Jack can read all about it in his new Vizard book.
 
Just start by takeing em all apart and cleaning good and checking the seats, both and the throttle rods.

Hay I read all the books. hehe
 
If the pistons do not fall out of the bells at a similar rate, then you could end up with a pair of carbs that are hard to balance and one set of cylinders lean/rich on acceleration. When you get the carbs, let us know and we can give more detail on what to check for.
 
I am going on vacation this weekend, and told the PO not to ship until next week. So when I get them in about 2 -3 weeks I will clean them and add oil to the dashpots and see how free the pistons fall. If not well I will try and swap the pistons to see if that improves them, and I will be posting their state of health and asking for help. Trevor, at least these both have the vacuum port. ANd I can always hope that if they are not a healthy pair, the front one is ok, so I could only change out 1 carb to give me a matched pair.....
 
[ QUOTE ]
If the pistons do not fall out of the bells at a similar rate, then you could end up with a pair of carbs that are hard to balance and one set of cylinders lean/rich on acceleration. When you get the carbs, let us know and we can give more detail on what to check for.

[/ QUOTE ]


hmmm, could this be adjusted using the atmospheric vent holes?

mark
 
[ QUOTE ]
[ QUOTE ]
If the pistons do not fall out of the bells at a similar rate, then you could end up with a pair of carbs that are hard to balance and one set of cylinders lean/rich on acceleration. When you get the carbs, let us know and we can give more detail on what to check for.

[/ QUOTE ]


hmmm, could this be adjusted using the atmospheric vent holes?

mark

[/ QUOTE ]

Interesting idea, use a restictive gasket on one that rises or falls too fast? Could probably also vary the quantity and type of dash pot oil.
 
i would like to hear more about this test. I would imagine that how fast or slow the piston falls is not as critical as both being the same. another question is does it make a difference if the vents are inside the aircleaner assembly or outside the assembly, such as the pair of short air horns that i have?

mark
 
Well the test is as follows , and this is a quote from the SU book on carb tuning,
"Clean inside the suction chamber and piston rod guide using petrol or methlated spirit. Refit the damper assembly and washer. Seal the transfer holes in the piston assembly with rubber plugs or Pasticine and fit the assembly to the suction chamber. Invert the complete assembly and allow the suction chamber to fall away from the piston. Check the time this takes, which should be 3 to 5 seconds for HS2 type carbs of 1 1/4 in bore or 5 to 7 seconds for larger carbs. If the time taken is excess of that quoted the cause will be thicj oil on the piston rod, or an oil film on the piston inside the suction chamber. Remove the Oil from the points indicated and recheck. "

SU does not mention any other cause for longer times. I guess they figure that with wear the times should decrease. I would assume that with faster falling dampers a thicker oil could be used to slow the rise and fall of a damper when the car is operating. Using this logic, light oil should be used when the fall time is correct and a thicker , say 10-50, oil could be used for faster damper fall times. If one damper is faster than the other it would be interesting to use different oils in each dashpot to match them. Respect to you Trevor and Jack, but this seems like a better idea than using emery cloth, as long as the carbs speeds can be matched.

Back to my new/old carbs, I am hoping that they were owned by a DPO, and perhaps he switched the dampers in the carbs and all that is needed is to re match the dampers and bells. Oh well time will tell. I will keep you informed in a couple of weeks when I get them and have had a chance to play with them. cheers
 
I thnk it would be difficult to maintain the correct oil in each. The difference in the two is not extremely important unless there is an extreme difference. If there is an extreme difference, then the first thing to try is switching the pistons. If that doesn't work you need to see what is causing the hang up in the slow one. Often it is only gummed up oil, but sometimes it is a burr caused by dropping during reassembly. Once you set down and play with them for awhile you should be able to determine what is causing the problem. On one of my previous sets, I think the piston came from another set of carbs., but after tinkering (at one point involving very fine emery cloth) I managed to get the fall rates within less than a second of each other.

But Woodie, I'm no SU expert. I've only rebuilt 2 sets, and I did just as you are doing... used a manual, observation, logic, and advice from others.

Keep us posted.
 
Back
Top